Support ADT types in type info reflection
Tracking issue: rust-lang/rust#146922 `#![feature(type_info)]`
This PR supports ADT types for feature `type_info` reflection.
(It's still a draft PR, with implementation in progress)
Note that this PR does not take SemVer into consideration (I left a FIXME comment). As discussed earlier ([comment](https://github.com/rust-lang/rust/pull/146923#discussion_r2372249477)), this requires further discussion. However, I hope we could get an initial implementation to land first, so we can start playing with it.
### Progress / Checklist
- [x] Struct support.
- [x] Enum
- [x] Union
- [x] Generics
- [ ] ~Methods~ Implemented and to be implemented in other PRs
- [ ] ~Traits~ Implemented and to be implemented in other PRs
- [x] Rebasing PR to `main` branch
~~(It's currently based on PR rust-lang/rust#151123, so here's an extra commit)~~
- [x] Cleanup and Rebase.
- [x] Fix field info for references. (see [comment](https://github.com/rust-lang/rust/pull/151142#discussion_r2777920512))
r? @oli-obk
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